The present invention relates generally to the field of medical devices and methods and, more particularly, to a device and method for infusion of therapeutic agents into the body.
Six million Americans have diabetes. There are two major types of diabetes, Type I (Insulin Dependent Diabetes) and Type II (Insulin Independent Diabetes). About 80% of the diabetics are Type II and about 20% are Type I. All of the Type I diabetics require insulin treatment and many of the Type II patients also require insulin.
In order to achieve good glucose control the insulin administration must be matched to the blood glucose availability. Blood glucose levels are influenced by food intake, exercise, stress and illness. It has become evident that by matching the insulin dosage to glucose intake and achieving good glucose control the long-term complications of diabetes can be diminished. In 1986 the cost of treatment of diabetic complications was $16 billion dollars or 4% of the federal budget. These long-term complications of diabetes include renal failure, cardiovascular disease, blindness and amputations. Continuous insulin infusion using portable infusion pumps has become an important method of treating insulin dependent diabetics. In 1985, there were about 10,000 insulin pumps in use. The use of pumps to deliver insulin has a number of advantages over the more widely used method of insulin injections. Most insulin pumps in use today are small in size, about as large as a deck of cards. The primary advantage in using such pumps for insulin administration is that insulin can be delivered at a slow, basal rate, continuously and at a higher rate after food consumption. This pattern of insulin administration mimics more closely the normal secretion pattern of the human pancreas than does the insulin absorption from injected insulin. Another advantage is that the rate can be varied to meet changing demands and unplanned events, such as a change in meal time or size, can be compensated for easily. To even begin to approach the insulin patterns achievable by insulin infusion pumps using injections requires at least 4 injections per day. Because continuous infusion of a drug offers the advantages of maintaining more constant drug levels than by bolus administration, swings in concentration depending on the absorption and elimination characteristics of the drug can be compensated for more easily. Maintaining a constant low level of a drug becomes especially important in drugs which must have a certain minimum concentration level in the body to be effective but become toxic at higher levels. Bolus administration of drugs with a low therapeutic index may expose patients to transient toxic levels of a drug to maintain the necessary effective lower level. Availability of a more permanent access device provides the option of infusing a greater variety of drugs to maintain constant low effective concentration while minimizing the exposure of the patient to toxic levels of a particular drug. Examples of these types of drugs for which this feature would be useful include chemotherapy agents used to treat cancer and immunosuppressive drugs used to prevent post-transplant tissue rejection.
One example of a continuous insulin infusion pump contains an insulin reservoir which is connected to a long tube having a needle at the opposite end. The needle is inserted into the subcutaneous tissue, usually the abdomen, and taped in place. There are several problems associated with this method, which is generally known as the subcutaneous access method. First, the mere presence of the needle can be painful. Any bumps or abrasions to the area of the needle insertion cause additional pain. Since the needle is secured in place only by tape, there is the possibility that it will become dislodged without the patient even being aware of the dislodgement. This can result in the patient not receiving the needed medication. For diabetics whose insulin is not being administered, such an occurrence can lead to ketoacidosis and coma.
In addition, some people are allergic to the tapes used to hold the needle in place. Yet further, certain patients are prone to infection of the needle insertion sites. This may be due to a lowered resistance to infection or to poor insertion procedures. Unfortunately, such infections are not uncommon. As an example, in one study lasting 11/2 years 29% of the patients involved developed an infection serious enough to require antibiotic therapy.
As an alternative to the subcutaneous needle, a catheter like device consisting of a short plastic tube having a needle stylus inside can be employed. The catheter/needle combination is inserted into the tissue and the needle removed, thereby leaving the catheter in place. An advantage of a catheter over a hollow needle is that a catheter is less painful in place and more body compatible. However, whether the access device for the external pump is a hollow needle or catheter/needle combination, they can be left in place for only a few days before a new needle or catheter must be inserted because of the risk of infection entering the body through the open lumen or around the exterior surface of the device. Also, with either the hollow needle or catheter/needle combination, absorption of the therapeutic agent is limited due to the limited body area contiguous to the lumen opening. This is not a significant problem in vasculr infusion situations because of the ability of the bloodstream to carry and diffuse the therapeutic agent throughout the body. Moreover, many of the other above-described disadvantages associated with the hollow needle are still present when a catheter is used as a the access device.
In addition to external mounted infusion pumps, there are also permanently implantable infusion pumps. Characteristically, such pumps contain reservoirs for insulin which must be filled periodically from outside the body by injecting insulin into the reservoir through a septum which is placed near the skin surface. So far, these devices have been used only on an experimental basis. There are at least several potential disadvantages in an infusion system of this type. For example, in the case of a malfunction, major surgery would be required to remove the pump. There is also the risk of a malfunction of the type which would cause a "run away pump." As opposed to an external pump which can be simply disconnected by the patient or a family member, it would require a trained professional to stop a malfunctioning implanted infusion pump. Further, changing the operational programming in an implantable pump is inherently more complicated than an external mounted pump because it must be done remotely. Of added concern are the infections which can occur in the body area or pocket formed around the pump. The onset of such infections usually necessitates removal of the pump. Moreover, if the catheter leading from the pump is placed inside the vascular system there is the undesired potential of decreased flow rate due to thrombus formation at the end of the catheter.